Air to air heat exchanger



FIG. 1 is a front and right side perspective view of an air to air heat exchanger showing our new design;

FIG. 2 is a top plan view thereof;

FIG. 3 is a front elevational view thereof;

FIG. 4 is a right side elevational view thereof;

FIG. 5 is a rear elevational view thereof;

FIG. 6 is a left side elevational view;

FIG. 7 is a bottom plan view thereof; and,

FIG. 8 is a front and right side perspective view thereof on a reduced scale, shown in use, the broken line showing of environmental structure is for illustrative purposes only and forms no part of the claimed design. 

The ornamental design for an air to air heat exchanger, as shown and described. 